The present invention relates to a novel enzyme, in particular a levodione reductase (hereinafter referred to as LR), a process for producing the enzyme, and a process for producing (4R, 6R)-4-hydroxy-2,2,6-trimethylcyclohexanone (hereinafter referred to as actinol) from (6R)-2,2,6-trimethylcyclohexane-1,4-dione (hereinafter referred to as levodione) utilizing the enzyme.
Actinol is an important intermediate for the production of zeaxanthin. European Patent Application Nos. 98115564.1 and 99115723.1 disclose processes for the manufacture of actinol. Such processes include contacting levodione with a microorganism selected from Cellulomonas, Corynebacterium, Planococcus and Arthrobacter which is capable of the selective asymmetric reduction of levodione to actinol. The resulting actinol from the reaction mixture is recovered therefrom. Corynebacterium aquaticum AKU611 (FERM BP-6448) was found to be one of the best microorganism strains for this purpose.
Corynebacterium aquaticum AKU611 has the following taxonomical properties:
Moreover, the strain Corynebacterium aquaticum AKU611 was identified as having these characteristics based on assimilation of various carbon sources by the Biolog System (Biolog Inc., Hayward, Calif., see also Nature Vol. 339, 157-158, May 11, 1989) as follows: 96-well microtiter-plates were inoculated with Corynebacterium aquaticum cells and incubated for 24 hours at 28xc2x0 C. Each well contained one of the 96 types of carbon sources in BUGM+B medium (Biolog Universal Growth Media+Blood) (Biolog Inc.).
After incubation, the strain showed the following assimilation of carbon sources:
In the table, xe2x80x9c+xe2x80x9d indicates that the carbon source was assimilable and xe2x80x9cxe2x88x92xe2x80x9d indicates that it was not assimilable.
The alpha-numeric codes set forth above are defined in the table below:
One embodiment of the present invention provides an isolated and purified enzyme having levodione reductase activity wherein the enzyme includes the following physico-chemical properties:
(a) a molecular weight of about 142,000 to about 155,000xc2x110,000;
(b) a nicotinamide adenine dinucleotide (AND/NADH) co-factor;
(c) a substrate specificity for levodione;
(d) an optimum temperature of about 15xc2x0 C. to about 20xc2x0 C. at a pH of about 7.0;
(e) an optimum pH of about 7.5; and
(f) wherein the enzyme is activated by K+, Cs+, Rb+, Na+ and NH4+.
A process for producing an enzyme having levodione reductase activity is also provided wherein the enzyme has the following physico-chemical properties: a molecular weight of about 142,000 to about 155,000xc2x110,000, a nicotinamide adenine dinucleotide (AND/NADH) co-factor, a substrate specificity for levodione, an optimum temperature of about 15xc2x0 C. to about 20xc2x0 C. at a pH of about 7.0, an optimum pH of about 7.5, and wherein the enzyme is activated by K+, Cs+, Rb+, Na+ and NH4+. This process includes cultivating cells of a Corynebacterium in an aqueous nutrient medium under aerobic conditions; and disrupting the cells to form a cell free extract containing the enzyme.
A process is also provided for producing actinol from levodione. This process includes forming a reaction mixture containing levodione and (i) a cell-free extract derived from Corynebacterium containing a levodione reductase or (ii) a levodione reductase having the following physico-chemical properties: a molecular weight of about 142,000 to about 155,000xc2x110,000, a nicotinamide adenine dinucleotide (AND/NADH) co-factor, a substrate specificity for levodione, an optimum temperature of about 15xc2x0 C. to about 20xc2x0 C. at a pH of about 7.0, an optimum pH of about 7.5, and which enzyme is activated by K+, Cs+, Rb+, Na+ and NH4+; adding a reduced form of nicotinamide adenine dinucleotide to the reaction mixture; and isolating actinol from the reaction mixture.
An embodiment provides an isolated and purified levodione reductase derived from Corynebacterium aquaticum AKU611 (FERM BP-6448) cells having the following properties:
(a) a molecular weight of about 142,000 to about 155,000xc2x110,000,
(b) a AND/NADH cofactor;
(c) a substrate specificity for levodione;
(d) an optimum temperature of about 15xc2x0 C. to about 20xc2x0 C. at a pH of about 7.0;
(e) a optimum pH of about 7.5; and
(f) the levodione reductase being activated by K+, Cs+, Rb+, Na+ and NH4+.
A purified LR sample prepared according to the Examples presented below has the following physico-chemical properties:
1) Enzyme Activity
The novel LR of the present invention catalyzes the reduction of levodione to actinol in the presence of a co-factor according to the following formula:
Levodione+NADH⇄Actinol+AND
It has been determined that the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) does not work as an electron donor in this reaction system.
A standard enzyme assay for detecting levodione reductase enzyme activity has been used in the present invention. This assay is exemplary of a number of recognized assays for detecting LR activity. The LR activity assay used herein is set forth below:
A basal reaction mixture having a total volume of 500 xcexcl includes 100 xcexcl of 1 M potassium phosphate buffer (pH 7.0), 20 xcexcl of 8 mM NADH in 0.2 mM KOH, 10-40 xcexcl of the enzyme solution (i.e., levodione reductase), and water up to a total of 500 xcexcl . This reaction mixture was incubated for 1 minute at 37 xc2x0 C. Then, 2 xcexcl of 0.5 M levodione solution were added to give a final concentration of 2 mM, and the whole mixture was incubated for 1 minute at 37 xc2x0 C. The levodione reductase enzyme activity was monitored with the decrease of the absorbance of NADH at 340 nm.
In the present invention, one unit of the levodione reductase enzyme activity is defined as the amount of the levodione reductase which catalyzes the oxidation of 1 xcexcmole of NADH per minute.
The AND, NADH, and NADPH used in the example set forth herein were obtained from Oriental Yeast (Tokyo, Japan). The protein concentration was determined using a Bio-Rad protein assay kit (Bio-Rad Laboratories (Hercules, Calif.)
2) Molecular Weight
The molecular weight (MW) of the enzyme was measured with a gel filtration HPLC column Cosmosil 5Diol-300 (nacalai tesque: Kyoto-fu, Japan). The apparent molecular weight of the (whole) enzyme was calculated to be about 142,000 to about 155,000xc2x110,000 in comparison to the molecular weight marker proteins: LMW+HMW gel filtration calibration kit, Amersham Pharmacia Biotech (SE-75184 Uppsala, Sweden); ferritin (MW 440,000), aldolase (MW 158,000), bovine serum albumin (MW 67,000), ovalbumin (MW 43,000), and ribonuclease A (MW 13,700).
When a purified sample of the enzyme was subjected to SDS-polyacrylamide gel electrophoresis (SDS-PAGE), a single band with a molecular weight of about 36,000xc2x15,000 was observed in comparison to the molecular weight marker proteins: LMW Electrophoresis calibration kit, Amersham Pharmacia Biotech; bovine serum albumin (MW 67,000), ovalbumin (MW 43,000), carbonic anhydrase (MW 30,000), soybean trypsin inhibitor (MW 20,100), and xcex1-lactalbumin (MW 14,400). These results indicate that the enzyme (i.e., levodione reductase) is composed of four homologous subunits. The values of the molecular weight of the whole enzyme (142,000-155,000xc2x110,000) and of each subunit (36,000xc2x15,000) were determined by the gel filtration column method and the SDS-PAGE method, allowed.
3) Co-factor
In the present invention, it was established that NADH could serve as a co-factor for this reductive reaction, but that NADPH could not.
4) Substrate Specificity
The substrate specificity of the enzyme was determined using the same enzyme assay method as described under 1, except that various solutions containing different substrates (2 mM final concentration in the reaction mixture) were used instead of levodione. It was shown that levodione was the only substrate for which the enzyme exhibited activity.
5) Optimum Temperature
The enzyme activities were measured at temperatures from about 2 to about 45 xc2x0 C. The optimum temperature of the enzyme activity was determined to be about 15xc2x0 C. to about 20xc2x0 C.
6) Optimum pH
The correlation between the enzyme activity and the pH values of the reaction mixture was determined using the same enzyme assay method as described in Table 1, except that various pHs and buffers were used and 40 xcexcl of 2.5M KCl solution were added to the reaction mixture. The optimum pH of the enzyme reaction was found to be 7.5 as set forth in Table 3 below.
7) Effect of Metal Ions
The effect of metal ions on the enzyme activity was investigated using the same enzyme assay method as described for Table 1, except that 100 xcexcl of 1 M Tris-HCl buffer (pH 7.5) were used instead of 100 xcexcl of 1 M potassium phosphate buffer (pH 7.0), and various metal solutions were added to the reaction mixture to give a final concentration of metal between 100 mM and 3 M. As a result, it was established that the enzyme activity was increased about 250-fold in the presence of 3 M RbCl and 1.8 M CsCl as shown in Table 4-1
8) Temperature Stability
the enzyme solution was treated at various temperatures for 10 minutes, and the remaining enzyme activities were measured using the same enzyme assay method as described for Table 1. Table 5, it was established that the enzyme was stable up to about 35xc2x0 C., and deactivated with increasing temperature, becoming completely de-activated at about 55xc2x0 C.
9) pH Stability
The enzyme was treated with 1 M buffers of various pHs for 10 minutes at 30xc2x0 C., and its remaining activity was measured using the same enzyme assay method as described in Section 1 above. The enzyme was found to be most stable in the pH range between about 8.0 and about 8.5 as shown in Table 6.
10) Michaelis Constant (Km) and Maximum Velocity (Vmax) Values
The Km and Vmax values of the enzyme were measured using levodione and actinol as the substrates. The basic enzyme assay method is the same as described under 1, but the substrate and the enzyme concentrations were varied. The Km and Vmax values against levodione as the substrate were 8.5 mM and 101.26 unit/mg, respectively. On the other hand, the Km and Vmax values against actinol as the substrate were 1.36 mM and 15.91 unit/mg, respectively.
The Km and Vmax values were calculated on the basis of the known Michaelis-Menten equation. Km is the concentration of the substrate that gives 50% of the Vmax of the enzyme reaction. The values provide a useful indication of the catalytic properties of the enzyme for the involved substrate.
11) Enzyme Purification Procedure
Purification of LR may, in principle, be effected by any combination of known purification methods, such as fractionation with precipitants, e.g. ammonium sulfate, polyethylene glycol, and the like, ion exchange chromatography, adsorption chromatography, gel-filtration chromatography, gel electrophoresis and salting out, and dialysis.
As set forth above, the LR provided by the present invention may be prepared by cultivating an appropriate microorganism in an aqueous nutrient medium under aerobic conditions, disrupting the cells of the microorganism, isolating, and purifying the LR from the cell-free-extract of the disrupted cells of the microorganism.
Microorganisms that may be used in the present invention include microorganisms belonging to the genus Corynebacterium which are capable of producing LR as defined hereinbefore. Functional equivalents, subcultures, mutants and variants of these microorganisms may also be used in the present invention.
In the present invention, a preferred strain of microorganism is Corynebacterium aquaticum, such as for example Corynebacterium aquaticum AKU611 (FERM BP-6448), a sample of which was deposited with the National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology, Japan, on Aug. 4, 1998, under the terms of the Budapest Treaty. European Patent Application Nos. 98115564.1 and 99115723.1 also disclose certain characteristics of this strain.
The microorganism of the present invention may be cultured in a nutrient medium containing, for example, saccharides such as glucose and sucrose, alcohols such as ethanol and glycerol, fatty acids such as oleic acid and stearic acid, or esters thereof, or oils such as rapeseed oil and soybean oil as carbon sources; ammonium sulfate, sodium nitrate, peptone, amino acids, corn steep liquor, bran, yeast extract and the like as nitrogen sources; magnesium sulfate, sodium chloride, calcium carbonate, potassium monohydrogen phosphate, potassium dihydrogen phosphate and the like as inorganic salt sources; and malt extract, meat extract, and the like as other nutrient sources. The cultivation may be carried out aerobically, normally for a period of about 1 to about 7 days at a medium pH of about 3 to about 9 and a cultivation temperature of about 10xc2x0 C. to about 40xc2x0 C.
In the present invention, isolation and purification of the LR from the microorganism after cultivation may be effected by, for example, harvesting the cells from a liquid culture broth by centrifugation or filtration. The harvested cells are washed with water, physiological saline or a buffer solution having an appropriate pH. The washed cells are suspended in the buffer solution and disrupted, for example, by means of a homogenizer, sonicator, French press, or treatment with lysozyme to give a solution of disrupted cells. The LR is isolated and purified from the cell-free extract of disrupted cells.
As set forth above, the LR provided by the present invention is useful as a catalyst for the production of actinol from levodione. The reaction of the LR-catalyzed reduction of levodione to actinol is conveniently conducted at pH values of from about 6.0 to about 9.0 in the presence of NADH in a solvent. As a solvent, any buffer which maintains the pH in the range of about 6.0 to about 9.0, such as Tris-HCl buffer, phosphate buffer, bis-tris buffer, HEPES buffer and the like, is suitable.
A preferred temperature range for carrying out the reaction is from about 2xc2x0 C. to about 30 xc2x0 C. The reaction usually gives the best results when the pH and the temperature are about 7.0 to about 8.0 and about 10xc2x0 C. to about 25 xc2x0 C., respectively.
The concentration of levodione in the solvent depends on the other reaction conditions, but in general is from about 1 mM to about 2 M, preferably from about 10 mM to about 100 mM.
The amount of the LR and NADH present in the reaction mixture depends on the other reaction conditions, but in general is in each case independently about 10xe2x88x924 to 10xe2x88x926 of the amount of the substrate. When a regeneration system of NADH from NAD is coupled with the above reaction system, the reaction proceeds more efficiently.
In the reaction, the LR may also be used in an immobilized state with an appropriate carrier. Any means of immobilizing enzymes generally known in the art may be used to immobilize the LR to a carrier. For instance, the enzyme may be bound directly to a membrane, granules or the like of a resin having one or more functional groups, or it may be bound to the resin through bridging compounds having one or more functional groups, e.g. glutaraldehyde. Such enzyme immobilizing reactions are described, for example, on pages 369-394 of the 2nd Edition of Microbial Enzymes and Biotechnology (Elsevier Applied Science 1990; Ed. W. M. Fogarty and C. T. Kelly).